1. Field of the Invention
The present invention relates to capacitive or electrostatic sonic emitters and more particularly to emitters which include a stator element and associated, movable emitter film useful for generating sonic output as a speaker device in response to an applied variable voltage.
2. Prior Art
The science of acoustics has long known of the utility of a movable electrostatic membrane or film associated with and insulated from a stator or driver member part of as a speaker and/or microphone device. Typical construction of such devices includes a flexible Mylar(tm) or Kapton(tm) film having a metalized coating and an associated conductive, rigid plate which are separated by an air gap or insulative material. An applied voltage including a sonic signal is transmitted to this capacitive assembly and operates to displace the flexible emitter film to propagate the desired sonic compression wave.
Two primary categories of electrostatic speakers exists, depending on the application and frequency of the sonic output. Single-ended speakers comprise a single plate, typically having holes to allow the sound to pass through. The film is suspended in front or behind the plate, and may be displaced out of contact with the plate by spacers. With ultrasonic emitters, the film has been biased in direct contact with an irregular face of the plate, whereby the film is allowed to vibrate in pockets or cavities. An insulative barrier of either air, plastic film or similar nonconductive material is sandwiched between the film and plate to prevent electrical contact and arcing. Typically, the plate and diaphragm are coupled to a dc power supply to establish opposing polarity at the respective conducting surfaces of the metalized coating and the plate. The capacitive relationship resulting from this configuration enables the electrostatic speaker to convert a variable voltage into sonic output as a compression wave.
The second primary category of electrostatic speakers is represented by the push-pull configuration. In this case, the speaker has two rigid plates which are symmetrically displaced on each side of a conductive membrane. With applied voltage, one plate becomes negative with respect to the membrane while the opposing plate assumes a positive charge. The transmission of a variable voltage to this capacitive assembly results in the reinforcing effects of push and pull on the membrane, thereby enhancing power output. Further details of theory and construction of common electrostatic emitter designs is found in Electrostatic Loudspeaker by Ronald Wagner, Audio Amateur Press, 1993.
Many years of directed research have developed a variety of technical improvements to this basic system, but the component definition has remained substantially the same. In particular, the rigid plate which operates as the stator or static element generally provides support to the film and supplies a conductive medium for application of the desired sonic signal. Typical materials used for plate construction include aluminum, copper plated circuit board, and similar materials well known in the art.
Although this pattern of material choice has generally served the purposes required for electrostatic speaker, the absence of substitute compositions has limited the variation in speaker applications. For example, audio systems are usually large in size to accommodate development of lower frequencies. Therefore, weight and geometric configuration become significant design factors. Furthermore, the requirement for rigidity and the known stiffness of accepted plate materials typically results in construction of the plate element as the load-bearing component of the speaker system. Taken together, the traditional limitations adopted by this history of plate development has diverted attention from exploring other options for electrostatic speaker design.